The present invention relates to a color picture tube having an improved inline gun, and particularly to an improvement in the electron gun for obtaining equal raster sizes (also called coma correction) within the tube.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen.
A problem that exists in a color picture tube having an inline gun is a coma distortion wherein the sizes of the rasters scanned on the screen by an external magnetic deflection yoke are different because of the eccentricity of the two outer beams with respect to the center of the yoke. Messineo et al. U.S. Pat. No. 3,164,737 issued Jan. 5, 1965, teaches that a similar coma distortion caused by using different beam velocities can be corrected by use of a magnetic shield around the path of one or more beams in a three gun assembly. Barkow U.S. Pat. No. 3,196,305, issued July 20, 1965, teaches the use of magnetic enhancers adjacent to the path of one or more beams in a delta gun, for the same purpose. Krackhardt et al. U.S. Pat. No. 3,534,208, issued Oct. 13, 1970, teaches the use of a magnetic shield around the middle one of three inline beams for coma correction. Yoshida et al. U.S. Pat. No. 3,548,249, issued Dec. 15, 1970, teaches the use of C-shaped elements positioned between the center and outer beams to enhance the effect of the vertical deflection field on the center beam. Murata et al. U.S. Pat. No. 3,594,600, issued July 20, 1971, teaches the use of C-shaped shields around the outer beams with the open sides of the members facing each other. These shields appear to shunt the vertical deflection field around all three beams. Takenaka et al. U.S. Pat. No. 3,860,850, issued Jan. 14, 1975, teaches the use of V-shaped enhancement members located above and below three inline beams and the use of C-shaped shields around the two outer beams. Hughes U.S. Pat. No. 3,873,879, issued Mar. 25, 1975, teaches the use of small disc-shaped enhancement elements above and below the center beam and ring shaped shunts around the two outer beams.
The inventions of all of the foregoing patents solve different raster correction problems. For example, in the Takenaka et al. patent the two V-shaped members and the two C-shaped members apparently correct for a raster pattern variation wherein the center beam has greater vertical deflection but lesser horizontal deflection than do the outer beams. The correction employed therefore decreases both the vertical and horizontal deflection of the outer beams, decreases the vertical deflection of the center beam and increases the horizontal deflection of the center beam. The four coma correction members of the gun disclosed in the Hughes patent correct for a raster pattern wherein the center beam has less deflection in both the vertical and horizontal directions than do the outer beams. This correction is made by decreasing both the vertical and horizontal deflection of the outer beams and increasing both the vertical and horizontal deflection of the center beam.
Another raster pattern problem has occurred in recently developed inline tubes utilizing a yoke having toroidal vertical deflection windings and saddle horizontal deflection windings which cannot be solved by any of the foregoing mentioned inline tube type coma correction arrangements. In this pattern, the central beam has lesser vertical deflection but equal or greater horizontal deflection than do the outer beams. The following described invention provides coma correction for such raster patterns by the use of a novel combination of correction members.